Polymeric linear polyesters are readily prepared by heating together dihydric alcohols or functional derivatives thereof and dibasic carboxylic acids or polyesterforming derivatives thereof such as acid halides, salts, or simple esters of volatile alcohols. Highly polymerized polyesters can be formed into filaments, fibers, films and the like which can be permanently oriented. The most widely known and most important commercially of the polymeric polyesters is that prepared by the condensation of terephthalic acid or dimethyl terephthalate and ethylene glycol.
Basically, there are two methods of producing polyester. The first and most common method is via the so-called "ester interchange" process wherein the dimethyl ester of the dicarboxylic acid is heated with and excess of glycol in the presence of an ester interchange catalyst to produce the bisglycol ester of the acid and alcohol, with excess ethylene glycol being distilled off. This product is then polycondensed with the elimination of glycol by heating the product with a catalyst at an elevated temperature and reduced pressures until the high molecular weight product is formed.
The second method employs the so-called "direct" or "acid esterification" process, whereby the dicarboxylic acid is heated with the glycol to form the diglycol ester and low oligimers thereof which can then be polycondensed by heating in the presence of a catalyst under reduced pressures to form a high molecular weight product. It is well recognized in the art that either of the major steps of either of these basic processes may be accomplished continuously or batchwise. With respect to the acid esterification approach, well known to be economically preferably over ester interchange, there are certain known advantages and disadvantages in the selection of either the continuous or batch for esterification with entirely different but equally important advantages and disadvantages in the selection of continuous or batch for polycondensation. Moreover there has been an understandable tendency to employ commercially systems selected on the basis of apparent continuity of operation. Logically a continuous esterification would seem to suggest a continuous polycondensation. A batch polycondensation would suggest a batch esterification. But the inherent disadvantages of batch esterification and continuous polycondensation, however well recognized in the prior art, by industries whose product end uses require the purity (low glycol ether content) of continuous esterification and the uniformity of batch polycondensation along with the economy of direct acid esterification (as opposed to ester interchange) have not led to a satisfactory functionally and cooperatively combined system employing the best selection of each phase, that is: continuous esterification and batch polycondensation which at the same time permits needed flexibility of operation.
Such a system would indeed be a significant advance in industries such as the textile industry where polymer quality is a most critical consideration.